Erlotinib (OSI-774) is a small molecule inhibitor of the epidermal growth factor receptor (EGFR) tyrosine kinase. OSI-774, like other EGFR-directed therapies, is associated with toxicities including skin rash and diarrhea. The molecular basis of these toxicities, and the basis for the high degree of interpatient variability in toxicity, has not been determined. Basal layers of both the epidermis and the gastrointestinal mucosa express EGFR, and EGFR signaling has been implicated in physiological regulation in these tissues. Skin toxicity in patients treated with EGFR-directed therapies may be of particular clinical relevance as several recent studies have suggested that skin rash may correlate with anti-tumor activity. We hypothesize that inhibition of EGFR-dependent signal transduction in non-malignant tissues may be responsible for OSI-774 toxicity, and may be a clinically relevant indicator of potential anti-tumor efficacy. This study will involve the administration of OSI-774 at a fixed starting dose in approximately 64 subjects with advanced solid tumors. Several analyses will be performed to assess potential causes of interpatient variability in toxicity. Length of a CA dinucleotide repeat polymorphism in the first intron of the EGFR gene has been strongly correlated with relative expression of EGFR. Specific Aim 1 will be a pharmacogenetic analysis, testing the hypothesis that length of this sequence polymorphism may serve as a predictor of OSI-774 toxicity in vivo. Specific Aim 2 will be a pharmacodynamic analysis, evaluating relative EGFR expression and activation, as well as expression and activation of the downstream mitogen activated kinases ERK1 and ERK2 in skin biopsies prior to and following administration of OSI-774. These analyses will test the hypothesis that CA dinucleotide repeat length correlates with relative degree of suppression of EGFR-dependent signaling. Specific Aim 3 will be a pharmacokinetic analysis, evaluating whether polymorphisms in the CYP3A5 metabolizing enzyme gene correlate with measures of OSI-774 metabolism. Together these analyses will characterize potential factors influencing interpatient variability in OSI-774 toxicity. A clear understanding of the basis of variability in the toxicity of OSI-774 and similar EGFR-directed agents might ultimately guide use of the currently available agents to patients most likely to benefit. Defining the basis of this toxicity could also promote the development of EGFR-directed agents that may avoid such toxicity or that may be effective in a broader spectrum of cancer patients.